A study confirms the connection between the AB blood type and an increased risk of blood clots.

Blood clots that form in the veins of the lower legs can pose serious health problems since they can break off and then get lodged in tiny arteries in the lungs, where they block blood flow and cause intense pain, difficulty breathing and even sudden death if blood can no longer pick up much-needed oxygen from the lungs. Being sedentary for long periods of time, such as during a long plane flight and after major surgery when patients are bedridden, or getting dehydrated can thicken the blood and increase the chances that a clot will form, and each year, about 2 million people develop such clots, known as deep-vein thrombosis (DVT, which is also called economy-class syndrome) and as many as 200,000 will die suddenly from complications of the condition.

Genetic factors play a major role in how likely a person is to develop clots; the most-well-known mutations include those in genes for Factor V Leiden and one of the clotting agents known as prothrombin. But researchers wanted to test what role blood types play in clotting risk, either alone or in combination with these genetic factors, and compared the effect that different blood types have on rates of DVT and clots in the lungs.

In the study, Dr. Borge G. Nordestgaard of Herlev Hospital at Copenhagen University Hospital and his colleagues analyzed blood-type and clotting-disorder information from 66,001 people participating in two Danish studies who were followed from 1977 through 2010.

After the lengthy follow-up, one trend emerged: people with the AB blood type showed a 4% higher risk of venous-blood clotting compared with those with the O blood type. Those with Factor V Leiden increased their risk of clotting events sevenfold compared with those with no variants of Factor V, and those with the clot-favoring prothrombin mutation showed an 11-fold increased risk of clotting incidents compared with those without the genetic aberration.

To understand what role each of the risk factors plays in contributing to cases of blood clotting, the scientists then calculate the population-attributable risk, or the proportion of deaths that could be prevented if each of the risk factors were eliminated. Because Factor V Leiden and prothrombin mutations are rare, the AB blood type proved to be the largest contributor, accounting for an estimated 20% of the risk of blood clots while Factor V accounted for about 10% and prothrombin another 1% of the risk.

That makes the AB blood type worth considering as a risk factor for clotting disorders since being overweight, with a BMI of over 25, accounts for about 16% of the population-attributable risk and smoking, which has been linked to higher risk of clotting, accounts for 6%.

What makes AB blood potentially prone to clotting? The blood type, in which both A and B factors appear on red blood cells, correlates with increased levels of von Willebrand factor in the blood, and higher concentration of this factor tends to promote clotting. According to the study authors, compared with people with type O blood, individuals with a non-O blood type have 25% to 30% higher plasma levels of von Willebrand factor.

“ABO blood type should be part of any screening for risk of venous[-blood clotting] — as 20% of this disease on a population level can be attributed to ABO blood type,” says Nordestgaard.

Not all individuals with AB type blood will develop clots, but the blood type may be a relatively easy way to identify those who may be at higher risk of DVT or pulmonary clots. Other behaviors, such as smoking and taking birth-control pills, can also increase the risk of clots, so even for those with O type blood and who aren’t genetically at higher risk of clots, it helps to keep blood circulating and at the right viscosity to avoid any potential problems. Staying hydrated and stretching your legs and arms by moving regularly can prevent blood from thickening and inhibit the clumping together of clotting factors — especially on long trips or after being bedridden. Every movement helps.